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The wording is poor and unclear, unfortunately, and does not appear to have been made any clearer in any errata - the Adventurer's Vault updates and errata does not mention Herbal Poultice.

It does not appear to acknowledge the possibility that someone would spend more than one healing surge. There is no particular indication for whether the author meant you get an additional 2 hit points for each healing surge, or just an extra 2 hit points for spending any at all!

You should talk to your group about it.

Considering the tiny bonus of 2hp though, for a one-rest single-target potion, I would certainly allow it to apply to each healing surge.